1. Field of the Invention
This invention relates to a device for controlling a physical quantity set by a user, such as temperature or humidity, in a household electrical appliance such as an oven, a refrigerator, a washing-drying machine or the like, in accordance with the introduction to the main claim.
2. Description of the Related Art
In a household electrical appliance of the aforesaid type the physical quantity relative to it has to be controlled to an optimum level. This control is usually achieved by sensors or probes (such as an NTC or PTC resistor) which modulate an electrical signal of intensity corresponding to the quantity measured. These probes are connected to control means which, on the basis of the emitted signal, evaluate the value of the controlled quantity and compare it with the value set by the user. On the basis of this comparison, said control means act on usual regulating means (such as an electrical resistance element, a compressor of a refrigeration circuit, or a fan), to enable or disenable them in order to maintain the value of the measured quantity or to change it to the set value.
These probes are usually connected to analog/digital converters which, on the basis of the signal emitted by the probes, generate a binary numerical value which is fed to a corresponding microprocessor circuit (defining the control means) which makes the said comparison and acts on the regulating means in the aforedescribed manner.
Such a device with an analog/digital converter (usually of 8 bit type) has however a limited capacity for discriminating different levels of the controlled quantity. For example, in the case of an oven temperature, a device of the said type allows good temperature discrimination substantially between 0.degree. C. and 250-300.degree. C., but not beyond. If considerable precision is desired in discriminating higher temperatures (such as those often reached in pyrolysis ovens), the analog/digital converter capacity must be raised to at least 16 bits. However such a converter is of high cost, which considerably affects the overall cost of the temperature control device. Alternatively, this greater precision can be achieved by operating on the unit downstream of the converter, on the electronic circuitry associated with it and on its operative algorithm. However this solution is also costly and complicated, and negatively influences the cost of the finished device.
In addition, known devices exhibit problems in the form of thermal drift of the signals measured by their probes, this affecting the measurement accuracy and hence the appliance control precision. These problems appear in particular during the control of relatively low temperatures, of the order of some tens of a degree.